1. Field of the Invention
The present invention relates to semiconductor device having a backside electrode for solder mounting, which is formed on a back side of a p-type semiconductor substrate.
2. Description of the Prior Art
The backside electrode is formed on the back side of the substrate of the semiconductor device having the p-type semiconductor substrate such as a diode, a bipolar-diode and IGBT.
FIG. 2 is a schematic cross-sectional view of a part of a conventional semiconductor device. In this figure, reference numeral 51 is a p-type semiconductor substrate, and reference numeral 52 is a backside electrode formed on the p-type semiconductor substrate 51. The backside electrode 52 includes a first metal layer 53 of aluminum, a second metal layer 54 of molybdenum or titanium as a barrier metal layer, a third metal layer 55 of nickel solderable to a lead frame and the substrate and an outermost fourth metal layer of gold. First metal layer 53 of aluminum has a low barrier height against silicon and can lower Vf which is a voltage drop of the on state semiconductor device. Fourth metal layer 56 of gold is formed to prevent the oxidation of third metal layer 55 of nickel.
The backside electrode is formed by depositing the above mentioned four metal layers on the back side of the substrate and subsequently by subjecting to a heat treatment to form a good ohmic contact between first metal layer 53 of aluminum and the semiconductor substrate 51. The semiconductor device is mounted on the substrate in such a way that the semiconductor device is bonded to the backside electrode by soldering.
However, it has been found that when the substrate and the backside electrode are subjected to a heat treatment so as to form a good ohmic contact between the substrate and the backside electrode, interdiffusion between nickel of the third metal layer and gold of the fourth metal layer occurs and a portion of nickel metal deposits on the outermost surface of the fourth metal layer of gold. The deposited nickel is immediately oxidized by a small amount of oxygen contained in an inert gas employed in the heat treatment, thereby to form a nickel oxide film on the outermost surface of the backside electrode. The resultant nickel oxide film prevents the semiconductor to be soldered to the substrate. Thus, the subsequent scrubbing process, which consists of rocking the semiconductor substrate in the planar direction, must to be carried out so as to destroy the nickel oxide film and to remove. A problem is that the scrubbing process is time-consuming and needs the complicated mounting apparatus.
Also, there is another problem that when displacing gold of the fourth metal layer with silver so as to prevent the interdiffusion of the nickel and gold, oxygen contained in the ambient atmosphere diffuses rapidly into the fourth metal layer of silver and oxidizes the third metal layer of nickel. As a result, the fourth metal layer of gold is easily peeled from the third metal layer of nickel.
An object of the present invention is therefore to provide a semiconductor device which does not need the scrubbing process following to the heat treatment and has a high reliability.
In order to achieve the object described above, the semiconductor device of the present invention includes a p-type semiconductor substrate, and a backside electrode disposed on a back side of the semiconductor substrate and including multiple layers of metal, the backside electrode including, on the semiconductor substrate, a first layer of aluminum, a second layer of barrier metal, a third layer of nickel, a fourth layer of silver and a fifth layer of gold which are disposed in this order.
The backside electrode of the present invention includes the layers of silver and gold disposed on the layer of nickel in that order. The layer of silver disposed between the layers of nickel and gold provides a barrier to inhibit an interdiffusion of nickel and gold, thereby to suppress the interdiffusion during the heat treatment process in the range of 350 to 500xc2x0 C. Accordingly, the layer of silver can prevent nickel to diffuse and deposit on the outermost surface of the backside electrode to form nickel oxide film.
Furthermore, since the backside electrode of the present invention includes the layer of gold disposed on the layer of silver, the layer of gold can block the diffusion of ambient oxygen into the layer of nickel through the layer of silver. Accordingly, the layer of gold can prevent the layer of nickel from becoming oxidized.
Furthermore, the backside electrode of the present invention includes the layer of barrier metal which provides a barrier to inhibit the interdiffusion of aluminum and nickel. Accordingly, the layer of barrier metal can prevent nickel to form an alloy with the semiconductor substrate and also prevent aluminum to diffuse and deposit on the outermost surface of the backside electrode to form aluminum oxide film.
Also according to the present invention, the layer of barrier metal may be formed of molybdenum or titanium.
Also according to the present invention, the fourth layer of silver and the fifth layer of gold may have a thickness of more than 400 nm and 200 nm, respectively, and the thickness of the fourth layer may be two times larger than that of the fifth layer.